CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > OpenFOAM Post-Processing

Whatbs the meaning of variables in the computed results

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree1Likes
  • 1 Post By eugene

Reply
 
LinkBack Thread Tools Display Modes
Old   May 26, 2005, 03:15
Default Hi, All I'm a new user of Ope
  #1
New Member
 
MaJianfeng
Join Date: Mar 2009
Posts: 8
Rep Power: 8
yfjma is on a distinguished road
Hi, All
I'm a new user of OpenFoam. I just found her several days ago and was interested in its open and robust attributes. After plays with it, i think it has a bright promising if it's help documentation can be strengthened.
Now, comes my question:
I run the LES case:channel395 and get several results written as files in different directory,e.g. in directory 1000, there are files of P,U,u_0,k,.....etc.
My exact question is what's the meaning of U_0,Umean,R,Bmean.
Does Umean means the average U with time??

Can anybody help me??
yfjma is offline   Reply With Quote

Old   May 26, 2005, 04:25
Default Umean is a running average of
  #2
Senior Member
 
Gavin Tabor
Join Date: Mar 2009
Posts: 181
Rep Power: 8
grtabor is on a distinguished road
Umean is a running average of the velocity field and R is a running average of the 2nd moments (velocity variance av(u'u'). Bmean is a running average of the SGS stress tensor. By running average I mean a continuously-updated time average of the quantity.

Most of this is in createAverages.H, which is fairly easy to read once you get used to the syntax. I actually can't find where U_0 is being defined : I'm wondering if its redundant. I'll keep looking.

Gavin
grtabor is offline   Reply With Quote

Old   May 26, 2005, 05:06
Default U_0 and the other *_0 fields a
  #3
Senior Member
 
Eugene de Villiers
Join Date: Mar 2009
Posts: 725
Rep Power: 12
eugene is on a distinguished road
U_0 and the other *_0 fields are the old time values (i.e. timestep previous to U). They are necessary to restart calculations that use second order accurate schemes in time like Crank-Nicholson.

For the rest,"p" for a incompressible code is pressure devided by density. For a compressible code it is just the pressure (same for mean quantities). "pPrime2Mean" is the mean of the square of the deviatoric part (p-pMean) of the pressure and is a good indicator for noise. "nuSgs "is the subgrid-scale viscosity and "k" is the SGS turbulent kinetic energy. "nuTilda" is a working SGS viscosity that is only used with the Spalart-allmaras model.

Have I missed anything?
arunsmec likes this.
eugene is offline   Reply With Quote

Old   January 29, 2007, 13:55
Default Hello Does anyone know what
  #4
Senior Member
 
santos's Avatar
 
Jose Luis Santos
Join Date: Mar 2009
Location: Portugal
Posts: 213
Rep Power: 9
santos is on a distinguished road
Send a message via Skype™ to santos
Hello

Does anyone know what is the definition of the 9 components of Reynolds stress R in OpenFOAM?

Thanks!
santos is offline   Reply With Quote

Old   September 12, 2007, 17:24
Default Hi all, I was looking at w
  #5
Senior Member
 
kumar
Join Date: Mar 2009
Posts: 112
Rep Power: 8
kumar2 is on a distinguished road
Hi all,

I was looking at what the components of R is and how they are calculated in channelOodles.

In createAverages.H, R is created as a symmetric tensor. so the 6 components of R written out by the program are Rxx,Rxy,Rxz,Ryy,Ryz and Rzz. As Dr. Tabor remarks above , R represents the variance of velocity thus the components of R should ; Rxx=ave(u'u') , Rxy=ave(u'v') , Rxz=ave(u',w'), Ryy=ave(v'v') , Ryz=ave(v'w') and Rzz=ave(w'w'). so the dimensions of R should be [0 2 -2 0 0 0 0] .

But if you look at the output file of R generated by the solver (say channelOodles.C) , then its dimension is [ 0 1 -1 0 0 0 0]. Is this correct? Also in createAverages.H, R is calculated as sqr(U)-sqr(UmeanPtr()) . thus R seems to be a function of velocity squared and should have dimension of m^2/s^2.

and can someone tell me how to get u'u' from sqr(U) - sqr(Umean) . It seems to me this should be square(U - Umean)

Thanks a lot

kumar
kumar2 is offline   Reply With Quote

Old   October 15, 2007, 07:59
Default Hi Kumar, About your last q
  #6
Member
 
hadi tartoussi
Join Date: Mar 2009
Location: paris
Posts: 48
Rep Power: 8
hadi is on a distinguished road
Hi Kumar,

About your last question i can tell you the following (PLEASE SOMEONE CORRECT ME IF I AM WRONG):

-R is a running average of u'u'
so R=<u'u'>=<(u-umean)(u-umean)>
=<(u-umean)^2>
=<u^2>
=<u^2> - 2<u.umean> + <umean^2>
=<u^2> -2<u>.Umean + Umean^2
=<u^2> -2.Umean^2 + Umean^2
=<u^2> - Umean^2.

Cheers,
Hadi
hadi is offline   Reply With Quote

Old   October 15, 2007, 08:01
Default Hi Kumar, About your last q
  #7
Member
 
hadi tartoussi
Join Date: Mar 2009
Location: paris
Posts: 48
Rep Power: 8
hadi is on a distinguished road
Hi Kumar,

About your last question i can tell you the following (PLEASE SOMEONE CORRECT ME IF I AM WRONG):

-R is a running average of u'u'
so R=<u'u'>=<(u-umean)(u-umean)>
=<(u-umean)^2>
=<u^2-2u.umean+umean^2>
=<u^2> - 2<u.umean> + <umean^2>
=<u^2> -2<u>.Umean + Umean^2
=<u^2> -2.Umean^2 + Umean^2
=<u^2> - Umean^2.

Cheers,
Hadi
hadi is offline   Reply With Quote

Old   June 13, 2010, 22:46
Default
  #8
Senior Member
 
Jiang
Join Date: Oct 2009
Location: Japan
Posts: 186
Rep Power: 7
panda60 is on a distinguished road
Very good!
Thanks for engene, kumar and hadi's explanation.
hadi's explanation is especially excellent.
This thread shouldn't be forgot.
panda60 is offline   Reply With Quote

Old   November 12, 2012, 02:52
Default
  #9
New Member
 
Arun S
Join Date: Feb 2012
Location: Chennai, India
Posts: 23
Rep Power: 5
arunsmec is on a distinguished road
Send a message via Skype™ to arunsmec
Quote:
Originally Posted by eugene View Post
U_0 and the other *_0 fields are the old time values (i.e. timestep previous to U). They are necessary to restart calculations that use second order accurate schemes in time like Crank-Nicholson.
If it is necessary only for restarting calculations, why we have to store this data? I am doing LES and is experiencing difficulty with disk space. If I dont save this *_0 fields, I can save about 10 GB space. Would you recommend removing these fields?
arunsmec is offline   Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
How WHF is computed by Fluent ? cfdltt FLUENT 0 January 24, 2008 11:46
Whatbs the best way to visualize paticles from lagrangian solvers sek OpenFOAM Paraview & paraFoam 2 September 19, 2007 17:02
pressure is not only computed KtoTo CD-adapco 0 March 5, 2007 12:09
Whatbs best way to create new case jvn OpenFOAM Pre-Processing 2 June 20, 2005 21:19
how can i put the computed data into simulation ? cornerstone Main CFD Forum 4 October 3, 2002 19:39


All times are GMT -4. The time now is 11:39.